Results of the writer's observations on the development of a deep-sea Laminariaceous alga Streptophyllopsis kuroshioensis in culture are described herein. They fundamentally agree with those of other investigators on species in this order except Chorda filum in which the sporophyte has sterile hairs. The evidences that the development of gametophytes and young sporophytes of the alga observed is considered to occure in nature during winter to early spring are described herein.